1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to a power supply circuit of a microwave oven for using a low AC voltage relay to cut off a sudden current in the microwave oven.
2. Description of the Prior Art
Generally, in microwave oven an AC power source is supplied to a magnetron to radiate microwaves, and the radiated microwaves are absorbed in water contained in food to cook the food by a molecular friction heat. A circuit for driving the magnetron uses a high voltage transformer for boosting an AC voltage of 120/230volts. A sudden current is produced during the initial application of operation, electric power for starting the cooking operating due to a characteristic of an inductive load. The sudden current deleteriously affects the internal circuit parts which is liable to result in damage of the parts in several cases. In order to prevent the sudden current, a relay circuit for cutting off the sudden current includes the power supply circuit.
Referring to FIG. 1, a circuit of a microwave oven is formed such that a first door switch 12, a variable program control switch 14 and a normally-open contact point 16a-16c of a monitor switch 16 are serially connected between a first AC power line 10 and one end of a primary coil 42 of a high voltage transformer 40. A timer switch 22, a second door switch 24 and a relay contact point 26 are serially connected between a second AC power line 20 and the other end of primary coil 42. Also, a normally-closed contact point 16b-16c of monitor switch 16 is connected to a common connecting point 23 of timer the switch 22 and the second door switch 24 via a fuse 18. The relay contact point 26 is connected in parallel with a current limit resistor 28. A relay coil 30 is connected in parallel with the normally-closed contact point 16b-16c of the monitor switch 16. A lamp 32 is connected between the first AC power line 10 and 23 of the common connecting point 23 timer switch 22 and the second door switch 24. A fan motor 34 and a timer motor 36 are parallel connected between a common connecting point 13 of the first door switch 12 and the variable program control switch 14 and common connecting point 23 of the timer switch 22 and the second door switch 24. A drivemotor 38 is connected between a low AC voltage output node 35 of the fan motor 34 and the common connecting point 23 of the timer switch 22 and the second door switch 24. A magnetron driving circuit 50 is connected to the secondary side of the high voltage transformer 40.
Under the state that the door of the microwave oven is closed, the first and second door switches 12 and 24 assume an on-state. In association with the monitor switch 16, the normally-open contact point 16a-16c is in an on state and normally-closed contact point 16b-16c is off state. Upon setting a cooking time, the timer switch 22 and the variable program control switch 14 are on initially to form an AC closed-circuit by the first door switch 12, the variable program control switch 14, the normally-open contact point 16a-16c of the monitor switch 16, the primary coil 42 of the high voltage transformer 40, the current limit resistor 28, the second door switch 24 and the timer switch 22 at the initial time. Therefore, the sudden current produced from the primary coil 42 is confined by the current limit resistor 28, thereby blocking an electrical shock caused by the sudden current imposed upon the circuit device. Additionally, the AC voltage is also applied to the relay coil 30, but it takes hundreds of ms until the relay coil 30 is excited to make the relay contact point 26 assume an on state. As the result, the relay coil 30 goes to the on state after the sudden current produced at the initial time of supplying the electric power is dissipated.
However, the conventional relay coil is driven by the AC voltage of 120/230 volts. These voltage levels require a large number of coil turns, which incites a problem of requiring so long a time (e.g., 6 minutes) in winding the relay coil during a step of assembling the relay part of a microwave oven automation line. The increased operating time exerts an influence upon output of production per unit time with the consequence of increasing the burden of production cost incurred.
Furthermore, in order to wind lots of turns around the iron core of the relay coil, the coil diameter becomes too thin (e.g., 0.03 cm). The thin coil is highly apt to be disconnected to bring about an inferior relay coil. The inferior coil in turn leads the current to continuously flow through the current limit resistor 28 to generate heat from the current limit resistor 28, which is liable to cause a fire or otherwise damage the microwave.